The present subject matter relates to 2-quinoxalinol salen compounds and in particular 2-quinoxalinol salen Schiff-base compounds. In particular, the disclosed 2-quinoxalinol salen compounds may be utilized as ligands for forming complexes with cations.
Salen ligands have been of interest to a wide variety of chemists. In particular, these have been investigated in a variety of applications because of their ease of preparation and ability to form stable complexes. For example, copper (I) salen complexes have been investigated as antitumor agents and protein kinase inhibitors. Salen ligands and their complexes also have been applied as catalysts in a variety of processes including as catalytic scavengers of hydrogen peroxide and cytoprotective agents, in the catalytic oxidation of secondary amines, in enantioselective catalysts for asymmetric epoxidation of unfunctionalized olefins or as catalysts for ring-opening metathesis. As catalysts, these complexes have proven quite useful, in particular after incorporation into solid supports and for chiral or stereoselective reaction catalysis. Therefore, new salen ligands are desirable.
Salen ligands are the product of a salicylic aldehyde compound and an ethylene diamine compound, hence the name “salen” is derived. Therefore, under suitable reaction conditions new ethylene diamine compounds may be utilized to create new salen ligands. 2-quinoxalinol is an ethylene diamine compound that previously has not been utilized for forming salen ligands. Derivatives of 2-quinoxalinols are key intermediates as bioactive agents in agriculture, have been used in dyes, and have been key pharmaceutical or medicinal intermediates. Synthetic methods for the parallel synthesis of 2-quinoxalinols have been previously reported. These factors have served to peak interest in utilizing 2-quinoxalinols for the preparation of a new series of 2-quinoxalinol salen framework ligands based on a Schiff base synthesis. The development of preparative methods using solution-phase parallel synthesis is not only feasible, but also desirable in the development of a new series of metal complexing agents that could be screened for bioactivity, or used in the development of new catalysts or metal selective sensors or sensing materials, through the incorporation of a unique coordination site and a quinoxaline that should have high UV and fluorescent activity. Taking advantage of a solution-phase combinatorial approach allows for the preparation of a series of ligands with a variety of substitution patterns. Here, it also is demonstrated that 2-quinoxalinols exhibit regioselectivity in Schiff base synthesis of salen ligands, which permits formation of asymmetrically substituted 2-quinoxalinol salen ligands.